Magento2 : Dependency injection And Object manager

Dependency injection :
Magento 2 uses dependency injection.In simple terms, when object A requires object or value B to fulfill a function, then B is a dependency of A Or A is dependent on B. Basically it avoids creating object manually.
Constructor injection :

public function __construct(NameSpace\ModuleName\Model $modelFactory) {
        $this->_modelFactory = $modelFactory;
    }
Method injection :
public function processCommand(\NameSpace\ModuleName\Model $model) 
    {
        // processCommand Code
    }

di.xml : 
The di.xml file configures which dependencies to inject by the object manager. Each module can have a global and area-specific di.xml file. Magento reads all the di.xml configuration files declared in the system and merges them all together by appending all nodes. Pass parameter to constructor using di.xml

<type name="path_to_class_file">
        <arguments>
            <argument name="mainTable" xsi:type="string">table_name</argument>
            <argument name="resourceModel" xsi:type="string">Namespace\ModuleName\Model\ResourceModel\BlogPosts</argument>
        </arguments>
</type>

Now mainTable and resourceModel can use in class constructor

public function __construct(
        $mainTable,
        $resourceModel) {}

Override Class Block Model of Core using di.xml

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../../../../../lib/internal/Magento/Framework/ObjectManager/etc/config.xsd">
    <preference for="CORE_CLASS_PATH" type="NAMESPACE_CLASS_PATH" />
</config>

Object manager : 
The object manager is a Magento service class that helps instantiate objects at the beginning of the bootstrapping process.
Initiate object manager

$objectManager = \Magento\Framework\App\ObjectManager::getInstance();